Metal implants

ABSTRACT

A metal implant for use in a surgical procedure is provided with a surface layer that is integral with the metal substrate, and which incorporates a biocidal material. The surface layer may be grown from the metal substrate, by anodising, and the biocidal material incorporated in it by ion exchange. Alternatively the layer may be deposited by electroplating, followed by diffusion bonding so as to become integral with the metal substrate. In either case, silver is a suitable biocidal material; and both the release rate and the quantity of biocidal material should be low to avoid toxic effects on body cells. Electropolishing the surface before formation of the surface layer is also beneficial, and this may be achieved by electropolishing.

This invention relates to metal implants for use in surgical procedures,and in particular to the introduction of a biocidal material into suchimplants to suppress or control infection.

Various surgical procedures require the use of implants. For examplecancerous bone may be removed, in prosthetic surgery, to be replaced bya metal implant. Such an implant may for example be of titanium alloy,which is very strong and relatively light. To ensure a hard-wearingsurface the provision of a titanium nitride coating has been suggested.There is furthermore a risk of introducing infection when implantingsuch metal implants, and it has been suggested that metallic silvermight be electroplated onto metal implants, the silver being a biocidalmaterial that can control infection without causing toxic effects to thepatient. However such coatings, whether of titanium nitride or silver,may be undercut due to corrosion from body fluids, so that the coatingmay detach from the implant, which may can increase wear and causetissue damage.

According to the present invention there is provided an implant for usein a surgical procedure, the implant comprising a metal substrate and asurface layer that is integral with the metal substrate, the layerincorporating a biocidal metal deposited from a solution.

The invention also provides a method of producing such an implant.

Such an integral surface layer may be generated by growing the layerfrom the metal itself, for example by an anodising process; oralternatively by depositing the layer for example by electroplating,followed by diffusion bonding so that the layer becomes integral withthe metal of the implant. Anodising forms an adherent oxide layer,although if it is carried out in phosphoric acid then a phosphate may beformed. Such an adherent phosphate layer may also be modified to form ahydroxyapatite layer, which can stimulate bone growth.

The biocidal material should preferably be effective for at least 6weeks, preferably for up to 6 months after surgery, and the release rateshould be low to avoid toxic effects on body cells. Furthermore thetotal quantity of biocidal material is preferably also limited tominimize any toxic effects.

It is also desirable if the surface is highly polished before productionof the surface layer. This may for example be achieved byelectropolishing.

In principle, a range of different metals may be used for the biocidalmetal. In particular, if the layer is a metal layer deposited byelectroplating then it clearly must be stable to corrosion. Gold,platinum, iridium and palladium would be potentially suitable, althoughexpensive; silver is preferable as it is not particularly soluble inbody fluids due to the presence of chloride ions and the low solubilityof silver chloride. If the surface layer contains the biocidal metal inionic form, then a wider range of metals would be possible. In additionto the elements already mentioned, copper, tin, antimony, lead, bismuthand zinc might be used as ions combined into an insoluble matrix forexample of metal oxide or metal phosphate. The rate of release would becontrolled, in this case, primarily by the strength of the absorption ofthe metal ions in the matrix.

The metals that may be used to make such prosthetic implants aretypically a form of stainless steel, a titanium alloy, or acobalt/chromium alloy, although zirconium could also be used. Thestandard alloys for this purpose are titanium 90% with 6% aluminium and4% vanadium (British standard 7252), or chromium 26.5-30%, molybdenum4.5-7%, and the remainder cobalt (British standard 7252 part 4).

Preferably the implant is initially polished to provide a very smoothsurface. Both stainless steel (chromium/iron/nickel) and cobalt/chromiumalloy can be electro-polished using as electrolyte a mixture ofphosphoric acid and glycerine, or a mixture of phosphoric acid andsulphuric acid. Titanium alloy can be electro-polished using aceticacid, or a mixture of nitric and hydrofluoric acids. Alternatively theimplants might be subjected to a combination of anodic passivation withmechanical polishing, which may be referred to as electrolinishing, thisprocess removing the oxide that protects surface roughness, the surfaceat that point then being electrochemically re-passivated, so producing amirror-smooth finish. Various electrolytes are suitable for thispurpose, including nitric acid mixed with sulphuric acid, sodiumhydroxide, sodium phosphate, or sodium hydroxide mixed with sodiumnitrate.

After polishing the surface of the metal, either silver deposition orsurface conversion can take place. Considering surface conversion first,a layer of metal oxide or phosphate may be formed by anodising in asuitable electrolyte, so that the oxide or phosphate layer builds outfrom the surface of the metal. Biocidal metal ions can then be absorbedfrom an aqueous salt solution into the oxide or phosphate matrix, forexample the ions Ag⁺ or Cu⁺⁺. Cations of palladium, platinum or evenruthenium could be absorbed in a similar way. If desired, depositedsilver, platinum or palladium ions could then be converted to metal, ordeposited ruthenium ions converted to insoluble RuO₂, within the oxideor phosphate surface coating, this reaction being performed chemicallyor electrochemically or by light.

Considering now silver deposition, the coating should be thin to preventtoxic effects. A high degree of adherence to the underlying metal can beensured by first removing the surface oxide layer by anodic etching,followed by a brief reversal of polarity in the presence of appropriateions, so as to cover the surface with a thin coating of silver. This maybe repeated to ensure there are no pin-holes. The plating electrolytemay include hydrofluoric acid, or may be an alkaline cyanideelectroplating electrolyte. After deposition, the silver coating shouldbe diffusion bonded so as to form an inter-metallic layer, by heatingthe implant to an elevated temperature. Typically it should be heated toabove 800° C., preferably between 810° C. and 950° C., in an inertatmosphere for example of argon for a period of between 1 and 6 hours.This substantially eliminates the risk of coating delamination. Howeverwith titaniumbased implants the temperature must not exceed 850° C. astitanium would undergo a phase change from alpha to beta form above thistemperature.

In place of silver, other metals such as platinum or palladium may beelectro-deposited and then thermally treated in a similar fashion so asto form an inter-metallic layer.

The invention will now be further and more particularly described, byway of example only.

A hip implant is made of titanium alloy (Ti/Al/V). The implant iscleaned ultrasonically using first acetone as the liquid phase, and thena 1 M aqueous solution of sodium hydroxide, and is then rinsed inde-ionised water. The cleaned implant is then immersed in a stirred 12weight % solution of phosphoric acid, and is anodised for 2 hours at amaximum voltage of 10V and a maximum current of 10 mA/cm², so as to forma surface coating of titanium phosphate. It is then rinsed in de-ionisedwater again. The surface, which is initially pale grey, turns to adarker matt grey as a consequence of the anodising, with a slightlyyellow hue.

The implant is then immersed in a stirred 0.1 M aqueous solution ofsilver nitrate, and left for 2 hours. As a result of ion exchange thereis consequently some silver phosphate in the titanium phosphate coating.The implant is then ready to be implanted. During exposure to bodyfluids there will be a slow leaching of silver ions from the phosphatelayer, so that any bacteria in the immediate vicinity of the implant arekilled. Infection arising from the implant is therefore suppressed.

Experimental samples of this titanium alloy were cleaned, anodised toform a layer of titanium phosphate, and then subjected to ion exchangeto form silver phosphate, following the procedure described above. Onesample was placed in direct daylight for 110 hours; the exposed surfacebecame darkened as a result of this exposure to daylight, indicating theformation of silver metal by photo-reduction. The other sample wasimmersed in a solvent containing a mixture of 4 M nitric acid and 0.5 Msodium fluoride (equivalent to hydrofluoric acid) to dissolve thecoating. The dark grey surface coating was removed completely within 3minutes, leaving a silver-grey finish. The resulting solution wasanalyzed for the presence of silver by atomic absorption spectrometry,and the concentration of silver was found to be equivalent to an averagesurface loading of 73 μg/cm².

1. An implant for use in a surgical procedure, the implant comprising a metal substrate and a surface layer that is integral with the metal substrate, the layer incorporating a biocidal metal deposited from a solution.
 2. An implant as claimed in claim 1 wherein the integral surface layer is generated by growing the layer from the metal.
 3. An implant as claimed in claim 2 wherein the surface layer is generated by an anodising process.
 4. An implant as claimed in claim 3 wherein the surface layer comprises a metal phosphate.
 5. An implant as claimed in claim 2 wherein the biocidal metal comprises metal ions absorbed within the surface layer.
 6. An implant as claimed in claim 5 wherein the biocidal material comprises silver.
 7. An implant as claimed in claim 1 wherein the integral surface layer is generated by first depositing the layer and then subjecting the layer and the substrate to diffusion bonding so that the layer becomes integral with the metal of the substrate.
 8. An implant as claimed in claim 1 wherein the surface of the implant is highly polished before provision of the surface layer. 9-12. (canceled)
 13. An implant as claimed in claim 3 wherein the biocidal metal comprises metal ions absorbed within the surface layer.
 14. An implant as claimed in claim 4 wherein the biocidal metal comprises metal ions absorbed within the surface layer.
 15. An implant as claimed in claim 1 wherein the biocidal material comprises silver.
 16. An implant as claimed in claim 2 wherein the biocidal material comprises silver.
 17. An implant as claimed in claim 3 wherein the biocidal material comprises silver.
 18. An implant as claimed in claim 4 wherein the biocidal material comprises silver. 